This description relates to small-scale fabrication systems and methods.
Suspended structures can be fabricated for a variety of photonic, mechanical, and optomechanical elements such as beams, waveguides, ring and disk resonators, membranes, and high quality factor (Q-factor) electromagnetic cavities. The suspended device architecture is useful for applications that demand mechanical degrees of freedom or optical isolation from the environment, which may be a requirement, for example, in the fabrication of both optical and mechanical resonators. Some fabrication techniques for suspended structures are limited to specific material systems.
There is particular interest in photonic devices with dimensions comparable to the wavelength of light. With mode areas (A) and volumes (V) close to the fundamental diffraction limit, A=(λ/2n)2 and V=(λ/2n)3 respectively, these photonic devices have enabled the engineering of strong light-matter interactions and numerous emerging applications such as ultrasmall lasers, optomechanical devices, optical switching, and chemical sensing. Many practical applications require the operating frequency of the device to be in the ultraviolet to near infrared range. Thus, the critical device dimensions are on the nanometer and sub-micron scales. One strategy for achieving guiding, confinement, and manipulation of light is to fabricate nanometer and sub-micron scale suspended beam device architectures (also referred to as nanobeams). In a suspended nanobeam, light is guided and confined to the beam due to the refractive index contrast between the nanobeam material and the surrounding medium (typically air). A suspended nanobeam may be patterned into a one-dimensional lattice to generate a photonic band gap material, and photonic crystal cavities can be defined from such structures by introduction of a defect. Such nanobeam photonic crystal cavities (also referred to as nanobeam cavities) have exceptional figures of merit (i.e. ultra high Q-factor with ultra small mode volumes). These simple structures rival the best two-dimensional planar photonic crystal cavities in reported and theoretical Q/V values. Moreover, the evanescent field in these suspended nanobeam cavities decays from all facets of the beam, which should facilitate sensing and biosensing applications as well as techniques for the dynamic control of cavity resonances.
To date, suspended nanobeam waveguides and cavities, along with nanomechanical resonant structures, have been demonstrated in a variety of materials systems used in the integrated circuit and optoelectronic industries, including silicon, silicon oxide, silicon nitride, and III-V semiconductors. Such suspended structures can be fabricated by surface nanomachining techniques. For example, surface nanomachining may begin with a heterostructure comprising structural (top) and sacrificial (middle) layers (of different materials) supported by a substrate (bottom). An etch mask is defined on top of the structural layer using deposition, lithography, and pattern transfer techniques. The pattern defined by the etch mask is then transferred into the device and sacrificial layers using a top down anisotropic (i.e., sensitive to direction) etch. After removal of the etch mask, the sacrificial layer under the structure is removed using a selective (i.e., sensitive to different materials) and isotropic (i.e., insensitive to direction) etch, which results in a free-standing structure. Similar techniques can also be used in the fabrication of micron and nanoscale ring and disk resonators. A prerequisite for employing surface nanomachining is the thin film heterolayer structure described, such that an isotropic etch may realize free-standing structures. In the case of material systems like silicon-on-insulator (SOI), gallium arsenide/aluminum arsenide (GaAs/AlGaAs), or amorphous silicon nitride on silicon, fabrication of a heterolayer structure is typically possible due to well-developed thin film chemical vapor deposition techniques.